During strenuous physical activity, the human body requires a significant amount of fluid replenishment to replace lost fluids, in order to prevent dehydration. It is preferable for a person to be able to consume a small amount of fluid frequently while continuing to perform the activity, rather than having to stop the activity periodically to consume a larger amount of fluid. Hydration packs such as those disclosed in U.S. Pat. Nos. 6,364,168 and 6,422,439 have been developed to allow frequent replenishment. They contain a flexible reservoir with up to several liters of fluid, and a flexible hose allowing the wearer to drink without interrupting their activity.
There are a large variety of devices known in the art for carrying standard bicycle water bottles. These are typically worn on a person's belt or attached to a standard backpack. There are also other types of reservoir carriers, such as the the Beverage Container Belt disclosed in U.S. Pat. No. 6,598,770. For longer periods of activity, a hydration pack is often preferred over these devices, due to its larger capacity. Hydration packs are commonly available that contain up to 3 liters of fluid, whereas bicycle water bottles typically contain no more than 1 liter.
The hydration pack may be worn by a person hiking in a remote area without a nearby potable water supply, requiring the reservoir to contain enough fluid for the entire hike. The hiker must ration the fluid to insure it does not run out prematurely. This is typically accomplished by controlling how often, as well as how much fluid is consumed, each time a drink is taken.
A disadvantage of hydration packs of the current art is a lack of any indication of the amount of fluid remaining in the reservoir. The pack is normally worn on a person's back, with the reservoir contained entirely inside of the pack, requiring the person to stop their activity, then remove and open up the pack in order to visually examine the liquid level. Although the weight of the pack will change as liquid is consumed, this is not a reliable gauge of remaining liquid, especially for an inexperienced user of the hydration pack.
Another disadvantage is an inability to indicate how much time the remaining fluid is expected to last, or to provide advance warning when it is about to run out.
There is a broad range of techniques for measuring fluid level. Many require mechanical devices such as floats installed in contact with the fluid. Others sense properties of the fluid using probes that are in direct contact with it, such as the impedance sensing technique disclosed in U.S. Pat. No. 5,565,851. Still others use techniques not requiring direct contact with the fluid, such as the optical technique disclosed in U.S. Pat. No. 4,840,137, and the capacitance measurement methods of U.S. Pat. Nos. 4,295,370 and 6,472,887.
The capacitance method is frequently employed, due to the possibility of using sensors that are not in contact with the fluid and therefore not subject to contamination by the fluid. The current art includes methods with sensors exhibiting a continuous capacitance change proportional to fluid level, as disclosed in U.S. Pat. No. 4,383,444; as well as methods with multiple sensors to detect for the presence of fluid at several discrete levels, as disclosed in U.S. Pat. No. 4,003,259.
A disadvantage of continuous capacitance methods of the current art is their inability to compensate for changes in ambient capacitance, such as changes in location of objects in proximity to the sensor, without requiring electrostatic shielding or other methods which increase the cost of the system.
A disadvantage of discrete capacitance methods of the current art is the inability to resolve more liquid levels than the number of electrodes in the system.
Another disadvantage of discrete capacitance methods is the necessity of filtering out variations in measured liquid level due to movement of the reservoir, such as is disclosed in “QProx QT114 Charge-Transfer QLevel Sensor IC” Data Sheet.
A technique known in the art as dithering may be used to increase resolution of an analog signal, as disclosed in U.S. Pat. No. 6,016,113.